1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding sheets one by one from a plurality of loaded sheets, and an image forming apparatus provided with this sheet feeding apparatus.
2. Description of Related Art
A photocopier, printer, printing press and such related devices have been using a sheet feeding apparatus wherein multiple sheets loaded on a sheet feed platen are fed one by one by a sheet feed roller. The sheet feeding apparatus using a sheet feed roller is accompanied by problems wherein non-feed failure is caused by the slipping of the sheet feed roller or, depending on the paper quality or environment, multiple sheet feeding failures occur such that multiple sheets sticking to each other are fed out. Further, with the progress of wear on the sheet feed roller, such a sheet feed failure becomes serious. One of the biggest current requirements is to find out how to solve these problems resulting from the spreading of the use of photocopiers and printers, an increase in the type of sheets to be used, and an enhanced feed speed.
In an effort to solve the aforementioned problems, attention is currently focused on a sheet feeding apparatus known under the name of a pneumatic sheet feeding apparatus, wherein sheets are floated by blowing of air, and the floated sheets are adsorbed by an adsorption/conveyance mechanism, whereby these sheets are individually separated and are then fed out. In the pneumatic sheet feeding apparatus, air is blown to the side of a bundle of loaded sheets so that the sheets are floated. The topmost one of the floated sheets is adsorbed and conveyed by the adsorption/conveyance mechanism. When air is blown to the lateral portion of the sheets, air enters between sheets until a sufficient layer of air is formed. This ensures the effective separation of sheets into one sheet and positive sheet feeding at a high speed.
The pneumatic sheet feeding apparatus, however, has come across a new problem wherein the sheets cannot be adsorbed by the adsorption/conveyance mechanism due to the failure of the sheets being floated sufficiently close to the position of the final sheet, wherein the residual number of sheets is drastically reduced, when the sheets stacked on the tray are sequentially fed. The following describes this problem with reference to FIG. 13.
In FIG. 13, the airflow V1 is blown to the side of the loaded sheet bundle loaded on the loading platen 70, and the sheets are floated. When the sheet P1 in the topmost position of the loaded sheet bundle has reached the level L, the adsorption/conveyance mechanism 60 adsorbs and conveys the sheet P1. For the adsorption/conveyance mechanism 60 to adsorb the sheet P1, the sheet P1 must be floated from the lower side as an adsorption surface of the adsorption/conveyance mechanism 60 to the level L having a prescribed interval.
In the meantime, the loading platen 70 is controlled so that the sheet P1 of the topmost position is constantly located at a prescribed height. Then, when there are not many sheets remaining on the loading platen 70, the loading platen 70 spans the air blowing opening 44 forming the airflow V1, in the vertical direction. This will result in a decrease in the amount of airflow V1 blown into the clearance of the loaded sheets, especially in the amount of air blown into the lower side of the sheet P1 in the topmost position. Thus, the sheet floating power is reduced. Accordingly, the sheet P1 in the topmost position fails to float up to a prescribed level L, and adsorption failure occurs to the adsorption/conveyance mechanism 60, with the result that no-feed failure takes place. Various forms of measures have been taken to solve this problem in the conventional art.
The Unexamined Japanese Patent Application Publication No. Hei 10 (1998)-226436, discloses a pneumatic sheet feeding apparatus wherein air is blown to the leading edge of the loaded sheets by means of a blowing unit, and the sheet in the topmost position is adsorbed and conveyed by means of an adsorption unit. In this pneumatic sheet feeding apparatus, a rib extending in the sheet conveying direction is provided on a sheet platen, and air is fed in the clearance formed between the sheets and sheet platen so that sheets are floated.
The Claim 5 and FIG. 6 of the Unexamined Japanese Patent Application Publication No. 2000-198557, show a pneumatic sheet feeding apparatus provided with a fan for blowing air to the leading edge of the sheets, and a final sheet blowing fan for blowing air to the lower side of the sheet from below.
The Unexamined Japanese Patent Application Publication No. Hei 10 (1998)-226436, is effective for the sheet having up to a prescribed level of basis weight. However, this is not effective for the sheet of extra heavy weight as exemplified by an A3-sized paper having a basis weight of 300 g/m2. To be more specific, when air is blown from the leading edge of the sheet, the greater part of air goes to the space above the sheet rather than the clearance formed between the rib and sheet. Even if the air volume is increased and a certain amount of air enters the clearance between the rib and sheet, the air sucked from one of the end faces to the point below the sheet in the topmost position flows directly to the end face on the opposite side. The effective buoyancy for raising the sheets to the adsorption unit cannot be gained, even if sheets are floated slightly. Thus, a heavy sheet cannot be floated.
The Unexamined Japanese Patent Application Publication No. 2000-198557 is effective for the sheet having a certain basis weight. For the sheet of extra heavy weight as exemplified by an A3-sized paper having a basis weight of 300 g/m2, a large-sized final sheet blowing fan must be used to generate a considerable amount of air pressure. This fan must be installed below the sheet loading section. This will increase the overall dimensions of the apparatus. A large-sized fan is required only to float the final sheet, and this leads to a cost increase.